The app stores already block porn on their own initiative.

> I also wonder why smut literature (the best selling category of books on Amazon) seems to get a free pass.

It's popular with women and basically invisible to men.

Text has always been treated differently than images or video, partly for historical reasons and partly because regulating it runs straight into classic First Amendment landmines

Probably because some apps aren't NSFW apps but have it (Reddit)

Because people were so sick of their shit, and they already got their asses beaten so hard that they turned a fundamentalist city into an atheistic one. Banned in Boston used to be a thing. Boston itself got sick of that puritan bullshit.

They know that re-litigating that is a road to ruin because 'artistic merit' is so well tread a ground in literature.

Text just fundamentally isn't nearly as graphic as images/video.

Write the most sexually disturbing sentence you can come up with and it's going to be rather meh and possibly quite comical. And any of the gravity that it does have comes from the reader's ability to generate the visuals themself which is mostly out of reach for children who don't have the experience to necessarily know what's even being described.